1. Field of the Invention
The present invention relates to semiconductor lasers and more particularly to temperature compensation techniques for slope efficiency.
2. Description of the Prior Art
For most of the history of semiconductor lasers, both the current threshold Ith and the slope efficiency η decreased monotonically and generally exponentially with increasing temperature of the active material. With the advent of VCSELs, it has been possible to vary the tuning of the cavity resonance with respect to the peak of the optical gain in order to produce a non-monotonic variation in the Ith which is most simply approximated by a quadratic function. This technique generally referred to as “gain offset” is well known in the VCSEL field. The VCSEL design parameters are generally set such that the variation in threshold is minimized over the operating temperature range. Gain offset has been demonstrated to a lesser degree in edge-emitting DFB lasers. Gain offset is not applicable to Fabry-Perot semiconductor lasers whose effective cavity length is about 10 μm or more. In all these lasers, and in fact in all known semiconductor lasers, the slope efficiency still decreases monotonically with temperature and is only negligibly, e.g. only to 2nd order, affected by gain offset. The temperature variation of the slope efficiency dη/dT has been governed by quantum mechanical confinement of the electrons and holes in the active region.
In prior art devices, there has been no effective method for controlling temperature variation of the slope efficiency dη/dT. The only method utilized by prior art devices is to optimize the quantum mechanical confinement within the limitations of the material system. This approach is generally already used to its maximum practical limit. It only reduces the decay of slope efficiency with temperature, but its effect is limited by the availability of material structures that are consistent with the desired laser emission wavelength. Furthermore, optimizing for quantum mechanical confinement may involve compromises with other aspects of the laser such as electrical resistance or manufacturability.